1. Field of the Invention
The present invention claims priority from Japanese Patent Application Serial No. 2001-347226, filed Nov. 13, 2001, which is herein incorporated by reference.
The present invention relates to a wired circuit board and, more particularly, to a wired circuit board used for electric/electronic equipment.
2. Description of the Prior Art
A wired circuit board, such as a flexible wired circuit board, widely used for a variety of electric/electronic equipment, is formed, for example, by laminating an insulating layer of polyimide resin and the like on either side or both sides of a conductive layer of a copper foil and the like.
When parts are mounted on this wired circuit board, the parts are sometimes damaged by static electricity in the mounting process. For solving this problem, several methods have been proposed wherein a semi-conducting layer is formed on the insulating layer so that the charged electric charge can be immediately attenuated to a smaller potential value than a potential at which the electrostatic damage of the parts may be caused. More specifically, the proposed methods include, for example, the method of forming on the insulating layer a semi-conducting layer of semi-conducting polymer, such as polyaniline and polypyrrole; the method of forming on the insulating layer a semi-conducting layer of material packed with conductive minute particles such as carbon black; and the method of forming on the insulating layer the semi-conducting layer of metal.
However, those methods have the following disadvantages. First, the method using the semi-conducting layer of the semi-conducting polymer such as polyaniline and polypyrrole has the following disadvantage. When the semi-conducting layer of the semi-conducting polymer contacts acid or alkali in the process of the manufacture of the wired circuit board, its conductive characteristics are deprived. Due to this, the semi-conducting layer must be plated or subjected to organic rust proofing before it is formed on the insulating layer, then causing change in characteristics of the plated surface or in organic rust proofing.
Second, the method using a semi-conducting layer of material packed with conductive minute particles such as carbon black has the disadvantage that the semi-conducting layer of material packed with conductive minute particles is not adequate for precision instruments such as hard disc drive. This is because there is the possibility that the conductive minute particles of the semi-conducting layer may be mixed into the precision instruments to cause malfunction or damage of the precision instruments.
Finally, the method using the semi-conducting layer of metal has the following disadvantage. Although the semi-conducting layer of metal can provide significantly reduced surface resistivity (also can be cited as ‘sheet resistance’) and thus may provide the electrostatic protection measure, since the surface resistivity is reduced so significantly, possible malfunction of the mounted parts may be caused by such a semi-conducting layer.